1. Field of the Invention
The present invention relates to an apparatus to improve an image sharpness adaptively according to multi-video formats and a method thereof, and more particularly, to an apparatus to improve an image sharpness and a method thereof which can improve the image sharpness adaptively according to video formats of an input video signal and an output video signal using a plurality of band pass filters.
2. Description of the Related Art
An image-sharpness improving apparatus is an apparatus capable of presenting a vivid image by improving a sharpness of the image. In an image acquiring or transmitting procedure, a noise may be introduced, or deterioration of a picture quality, such as image blurring, may occur. In this case, the sharpness of an image can be improved by emphasizing edge components of the image, and this causes the picture quality of the image to be improved.
FIG. 1 is a block diagram illustrating a conventional apparatus for improving an image sharpness. Referring to FIG. 1, the conventional image-sharpness improving apparatus includes a format converter (F/C) 110, a band pass filter (BPF) 120, a first differentiator 130, a second differentiator 140, an amplifier 150 and an adder 160.
The format converter 110 is a kind of comb filter, which converts an input video signal to match a video format of a display device on which an image is to be displayed.
The band pass filter 120 passes only a preset frequency band of the converted video signal outputted from the format converter 110. The frequency band, which passes through the band pass filter 120, is changed according to a resolution of the input video signal and a video system which outputs the image, and thus the band pass filter 120 should also be changed according to the input video signal and the video system.
The first differentiator 130 detects edge portions of the image by differentiating an output signal of the band pass filter 120. The second differentiator 140 separates rising edges and falling edges from the edge portions detected by the first differentiator 130 by differentiating the video signal outputted from the first differentiator 130.
The amplifier 150 performs clipping and gaining of the secondly differentiated signal, and outputs a signal amplified with a preset amplification factor in order to adjust the image sharpness according to the video system.
The adder 160 inverts an output signal of the amplifier 150, and adds the inverted signal to an original signal (converted video signal) outputted from the format converter 110. Accordingly, an output signal of the adder 160 becomes smaller than the original signal just before the rising edge, and becomes larger than the original signal just after the rising edge. Also, the output signal of the adder 160 becomes larger than the original signal just before the falling edge, and becomes smaller than the original signal just after the falling edge. Accordingly, the edge portions of the image are emphasized, and thus the image having an improved sharpness can be obtained.
For example, in a general NTSC type video system having a horizontal resolution of 480i, the sharpness improving apparatus operates in a manner that it extracts a low-frequency band of 2.5 MHz through the band pass filter 120, and slightly emphasizes the edge portions by setting a low gain for the sharpness.
Specifically, in the NTSC type video system, a maximum resolution of the video signal is 4.2 MHz, and thus a high-frequency range over 4.2 KHz corresponds to noise components. In practice, home VTRs or LDPs have low frequency bands. For example, a Hi8 or S-VHS type VTR has a frequency band of about 5 MHz, a typical VHS type VTR has a frequency band of about 3 MHz, and an LDP has a frequency band of about 5 MHz. Accordingly, the sharpness improving apparatus should use the band pass filter 120 having a low-frequency band in order to prevent high-frequency noise components from being emphasized.
Also, in an analog video system having much interference caused by external noises, a gain of the amplifier 150 for the sharpness is set low so as to prevent even external noise components from being emphasized.
In an ATSC type video system, such as an HDTV, having a horizontal resolution of 1080i, the maximum resolution of the video signal is five times higher than that of the NTSC type video system having the maximum resolution of 22 MHz. Thus, the ATSC type video system uses the band pass filter 120 having a high-frequency band to an extent of about 12 MHz to improve the sharpness, and the gain of the amplifier 150 is set pretty high to sharpen the image. The digital video system is not greatly affected by the external noise even if the gain of the amplifier is heightened, and thus the gain of the amplifier is set high in order to sufficiently improve the sharpness.
Meanwhile, in a video system having a horizontal resolution of 1080P (Progressive), the horizontal resolution is more than 40 MHz, which is twice the horizontal resolution of 1080i, and thus the band pass filter 120 should use the high-frequency band of about 30 MHz.
Accordingly, the conventional sharpness improving apparatus cannot be compatibly applied to both the NTSC type video system and the ATSC type video system, but should be separately applied to the video system in accordance with a type of the video system.
For example, in the sharpness improving apparatus for an analog video system, since the sharpness of the signal having passed through the low-frequency band pass filter of 2.5 MHz is emphasized, the sharpness of the high-frequency band, which is to be emphasized with respect to the video signal for the HDTV system of a high picture quality, cannot be improved, but a low-frequency noise of the 2.5 MHz frequency band is emphasized instead. Also, if an analog NTSC video signal is inputted to the sharpness improving apparatus for the HDTV system, a signal having passed through the high-frequency band pass filter of 12 MHz is emphasized with a high gain, and this causes the image having an emphasized high-frequency noise to be outputted without emphasizing the edge portions of the image.
Consequently, the conventional sharpness improving apparatus has the problems in that it performs a proper operation with respect to a video format having a constant resolution, and thus cannot operate adaptively according to multi-video formats having different resolutions.